This invention relates generally to hydraulic control systems used on certain types of work machines for controlling the operation of various attachments or tools which mount to such machines and, more particularly, to a hydraulic control circuit which will sense the pressure to the attachment cylinder or actuator and maintain pressure thereto while allowing the remaining system pressure to remain above the maximum allowable attachment cylinder pressure without necessarily tripping, or at least minimizing fluid flow through, a pressure relief valve associated with the attachment hydraulic circuit.
In the case of certain types of work machines such as excavators and backhoe loaders, attachments or tools such as a thumb, grapple, rake and the like are often times pivotally mounted to the stick or lift arm assembly so as to enable the work machine to perform other functions and tasks. These attachments or tools are typically mounted to the stick or lift arm assembly by cooperatively engageable mounting means associated with both the lift arm assembly and the particular attachment or tool involved. Typically, one or more hydraulic pumps will be used to operate and drive the various hydraulic cylinders used to actuate and control the lift and/or tilt mechanisms associated with a particular work machine including the cylinders and/or actuators used for operating and driving the particular attachment or tool. Often times, depending upon the particular attachment or tool and the particular application or task being performed by such attachment or tool, the maximum system pressure to operate and drive a particular attachment or tool will be less than the maximum system pressure to operate and drive other functions of the work machine such as the boom and stick arm assembly associated with some work machines.
For example, in the case of a hydraulic excavator, such work machine will typically include a boom which is pivotally moved by a boom cylinder, a stick or lift arm assembly which is pivotally moved by a stick cylinder, and a bucket which is coupled to the stick or lift arm assembly and is pivotally moved by a bucket cylinder. In normal operation, the bucket, arm and boom are pivoted with respect to one another so that the bucket moves towards the excavator to scoop up earth or other material.
Hydraulic excavators are also used to lift heavy objects such as pipes, structural components and the like. When used to perform this secondary lifting function, a counteracting thumb attachment is often times pivotally mounted to the lift arm assembly to accomplish the grasping, holding and lifting function. The thumb attachment, like the thumb on a human hand, works in conjunction with the bucket and allows the excavator to grasp and hold odd-shaped items as well as loose materials. The thumb attachment is operated and driven by a separate hydraulic cylinder and, depending upon the particular linkage associated with the thumb attachment, the thumb is fully retractable to allow normal operation of the bucket and is likewise pivotally rotatable so as to follow the bucket path throughout most of the dump and curl cycle.
Typically, the maximum allowable cylinder pressure to operate and drive the thumb cylinder will be less than other maximum pressures to operate other work devices and/or functions. For example, a typical maximum thumb cylinder pressure is approximately 2500 PSI whereas the maximum system pressure to operate and drive, for example, the stick and boom cylinders is typically approximately 5000 PSI. As a result, the hydraulic circuit associated with the thumb attachment will typically include a pressure relief valve to protect the operation of the thumb cylinder, such pressure relief valve opening at approximately 2500 PSI. Since a hydraulic fluid flow will follow the path of least resistance, the attachments requiring the lowest load pressure are supplied with fluid first. If the pressure relief valve associated with the thumb hydraulic circuit is tripped or opened at 2500 PSI, hydraulic fluid enroute to other work devices and/or functions demanding a higher load pressure will be diverted through the opened pressure relief valve thereby dumping such fluid to tank and providing insufficient fluid flow to the higher load pressure demanding devices and/or functions. When this flow limiting situation occurs, controllability of the particular work devices such as the boom and stick assembly as well as controllability of bucket and swing motion is severely limited and the work machine may stall or incorrectly perform the particular function or application.
It is therefore desirable to provide a hydraulic circuit for controlling an attachment to a particular work machine such as the thumb attachment associated with a hydraulic excavator such that the attachment cylinder or actuator pressure can be maintained within limits at or near the maximum allowed attachment pressure while, at the same time, substantially bypassing and/or limiting fluid flow through the pressure relief valve associated with the attachment circuit such that the system pressure can rise above the maximum allowed attachment pressure to operate and drive other functions of the work machine.
Accordingly, the present invention is directed to overcoming one or more of the problems as set forth above.
In accordance with the teachings of the present invention, a hydraulic circuit for controlling an attachment to a work machine such as thumb attachment used on an excavator is disclosed wherein the hydraulic circuit senses the pressure associated with the attachment cylinder or actuator and when the attachment cylinder pressure reaches a predetermined value, a hydraulic signal is sent to the valve or actuator controlling fluid flow to the attachment cylinder so as to at least partially close such valve thereby maintaining pressure to the attachment cylinder yet substantially limiting fluid flow through the attachment circuit. Since fluid flow through the valve, actuator or other control device that drives the attachment cylinder circuit is substantially reduced, fluid flow through the attachment circuit is minimized thereby allowing the remaining system pressure to remain above the maximum allowed attachment cylinder pressure in order to operate and drive other devices and/or work functions associated with the particular work machine such as the boom, bucket, stick and swing functions of a hydraulic excavator. As a result, the pressure drop in the fluid supplied to the other devices and/or work functions while the attachment circuit is operable will be substantially reduced and the remaining system pressure will be allowed to rise above the maximum allowable pressure to operate the attachment. If the pressure relief valve associated with the attachment circuit is, in fact, tripped or otherwise opened due to the fluid pressure within such circuit reaching a maximum pressure for the attachment cylinder, fluid flow from the remaining system through such pressure relief valve will be minimized since fluid flow through the attachment circuit is minimized. This will not severely limit fluid flow throughout the rest of the system thereby allowing the system pressure to the other devices and/or work functions to remain substantially unchanged.
In order to accomplish this task, the present hydraulic control circuit includes an attachment cylinder or actuator, a signal operated valve or controller for controlling fluid flow to the attachment cylinder, appropriate fluid paths communicating the attachment cylinder or actuator with the signal operated valve or controller, a pilot pressure source, a fluid path communicating the pilot pressure source with the signal operated valve or controller, a pilot signal control device positioned in the fluid path of the pilot pressure source and having a port in communication with the attachment cylinder for sensing the pressure associated therewith, and appropriate pressure reducing valves to both control fluid pressure to the attachment control valve and, under certain operating conditions, to maintain a predetermined pressure differential between the signal ports of the control valve. The valve or controller used for controlling fluid flow to the attachment cylinder is operable between an open position allowing fluid flow to the attachment cylinder and a closed position restricting flow thereto. The pilot signal control device is responsive to the head pressure associated with the attachment cylinder such that when the attachment cylinder head pressure reaches a predetermined valve, a pilot pressure signal is outputted to the signal operated valve or controller to at least partially close such valve thereby locking the attachment in a certain position and maintaining pressure to the attachment cylinder while at the same time minimizing fluid flow both to the attachment cylinder and to the pressure relief valve associated therewith. This circuit configuration maintains the attachment pressure but limits flow thereto thereby permitting the overall system pressure to remain above the maximum allowed attachment cylinder pressure so as to allow other functions associated with the work machine to continue to be available.
Although the present hydraulic circuit will be discussed and explained with respect to controlling a thumb attachment used in conjunction with a bucket on a hydraulic excavator, it is recognized and anticipated that the present hydraulic control system can be incorporated into any work machine to control a wide variety of different attachments or tools in accordance with the teachings of the present invention.